For the manufacture of gypsum products such as gypsum wallboard, raw gypsum rock is mined, crushed and calcined at about 160.degree.-175.degree. C. During calcination, the gypsum changes from calcium sulfate dihydrate (CaSO.sub.4 2H.sub.2 O) to calcium sulfate hemihydrate (CaSO.sub.4 1/2 H.sub.2 O), also known as stucco. The calcined rock is crushed to a fine powder.
The gypsum powder is combined with water and starch to form a slurry. Other ingredients, such as fillers and accelerators, may also be added. Upon mixing, the stucco begins rehydrating to its original calcium sulfate dihydrate form. As shown in FIG. 1, gypsum slurry is applied to the top of a bottom paper sheet 14 and forms a gypsum core, and then a top paper sheet 16 is applied over the core. As the slurry sets, dihydrate needles form and interlock with the paper at the core-paper interfaces. As the board 18 moves along the conveyor line 20 in a continuous sheet, the board gains sufficient green strength that it can be cut to standard sizes, turned over and fed into a 600.degree.-800.degree. F. kiln 22.
Once the board 18 is placed inside the kiln 22, there is a severe danger of the needles or crystals recalcining at the core-paper interfaces. This phenomena, called "burning", prevents the paper from firmly adhering to the core and greatly impairs the quality of the finished wallboard. The starch slurry initially added to the gypsum paste is provided to prevent burning. As the temperature rises, the starch migrates to the core-paper interfaces along with steam generated from the excess water expelled from the core in the kiln. The starch forms a wet jelly which serves as a heat sink to protect the crystal needles from recalcining so they maintain a strong interlock with the paper layers.
Gypsum board manufacturers have devised different methods for preparing the starch slurry used in the production of gypsum board. Some manufacturers prepare large quantities of the starch slurry to fulfill a single day's requirement. However, this normally results in the preparation of more starch slurry than is actually used. Since the forms of starch used in the production of gypsum board are relatively costly, this method adds additional expense to the manufacture and overall price of the board. Further, any excess starch slurry must be disposed of. Although starch slurry is non-toxic and can be discarded, it is also edible and provides a food for rats and other vermin. Still further, the excess starch ferments and becomes malodorous.
To overcome these disadvantages, other manufacturers prepare small batches of starch slurry more frequently on a schedule designed to correspond with the requirements of the production process. This system requires careful monitoring of the process and may result in halting production if the starch slurry production is not properly timed. Further, the more frequently batches of starch slurry are prepared, the more likely it becomes that the slurry will vary from batch to batch. This complicates the production of gypsum slurry to the desired specifications and increases the potential for nonuniformity in the finished product.
Still further, an optimum time exists for the starch slurry to hydrate and be used after it is prepared. Often, this time is considerably less than the time required to use all of slurry prepared in the small-batch approach.
Thus, there remains a need for a system to enable gypsum board manufacturers to continuously prepare the proper proportions of water and starch to form a slurry consistent with and responsive to the rate required for admixture with gypsum particles to enable efficient operation of the gypsum wallboard manufacturing equipment and process.